(1) Field of the Invention
The present invention relates to a method of coating an object by electrostatically attracting a paint material from a spray gun thereto.
(2) Description of the Prior Art
A typical installation employed in a conventional electrostatic spray coating method of the above-noted type is shown in FIG. 5. In this installation, an electrode 14a attached to a spraying opening of a spray gun 14 is preliminarily impressed with a positive or negative high voltage by means of a high-voltage generator 21, such that the paint material as being sprayed from the spray gun 14 is charged and this carged material is attracted to an electrically grounded object 4 for coating the same.
In the above method; however, if the employed paint material is electrically conductive, for example, a metallic paint containing metal powder or a water-thinnable paint containing water as its major solvent which paint has been drawing attention as an environmentally non-polutant material in recent years, since the paint material is continuously fed from a tank 13 to the spray gun 14, there is possibility of short circuit from the electrode 14a via the paint material throughout the feed pipe and also at the tank 13. For this reason, the entire paint feed system extending from the tank 13 to the pray gun 14 needs to be insulated by grounding the same, thereby disadvantageously enlarging the insulator equipment.
Moreover, it is a common practice in the spray coating installation to connect the single spray gun 14 via a color selector valve 16 with more than ten tanks 13 containing paint material of different colors. In this case, the above problem is more conspicuous since insulation is necessary for all of the great number of feed units.
With regard to the above problem, the prior art has suggested one solution shown in FIG. 6. In this system, there is provided a relay tank 22 for receiving dripping of paint fed from the upstream system, and the received paint material which has been insulated in the course of dripping thereof is subsequently fed through the downstream system to the spray gun 14. This system has the advantage that the insulated portion may be limited to the downstream side system after the relay tank 22.
On the other hand, with the above system, problems remain that the insulator system needs to additionally include the relay tank and that a plurality of the same are needed if the spray gun is connected to many paint tanks. More particularly, for changing paint colors, in order to obtain good painting efficiency, the paint feed pipe downstream of the valve and the spray gun should be cleaned as quickly as possible for receiving the next color paint and thus it is desired for the color selector valve to be disposed as close as possible to the spray gun, which results in the requirement that a plurality of relay tanks must be independently disposed upstream of the color selector valve for receiving the respective colors of paint materials. This is extremely disadvantageous for the reduction of the insulator system.
Therefore, the primary object of the present invention is to provide an improved electrostatic coating method in which significant reduction of insulator system is enabled by renovating the method of forming the electric field for electrostatically attracting the sprayed paint material to an object to be painted.